Low profile lens valves of various design have become widely accepted for use in the mitral area, and long term wear has been satisfactory. Certain changes in design and materials in order to decrease the incidence of thrombus on the valves have caused increased wear. Current research is being directed towards decreasing thromboembolism from heart valves but at the same time maintaining wear characteristics. It is also an aim of the project to continue developing techniques to make the dog a suitable animal for prosthetic and transplantation research. In regard to this latter aim, techniques have been developed whereby survival of dogs for over one month with various types of prostheses in the mitral area has been increased to about 90%, so that significant long term observations can now be made. More specifically, we are especially interested in analyzing patterns of clot formation and tissue overgrowth on artificial heart valves, particularly from the standpoint of the factors that initiate this fibrin formation.